What should happen when a notification plugin that still has destination configs is removed

[hmpf]

I can see three obvious reasons for this happening:

1. That kind of notification is no longer supported and there is no replacement
2. That kind of notification is now handled by a different plugin with a different name, for instance old: "sms_as_email", new: "sms"
3. That kind of notification is now handled by a different plugin with the same name, but different data in DestinationConfig.settings (BLEAURGH).

Others?

[lunkwill42]

Well, dang. Doublepost #535

[hmpf]

I would prefer for each:

1. Delete DestinationConfigs and rows in the Media-table. Maybe with a managment-command or api call that wants a superuser?
2. One-time batch-conversion
3. One-time batch-conversion but I'd rather not support this, frankly.